Eleven-year old Mísa has the temperament of a proper boy and endless problems with the grown-ups. It is no wonder that he longs to become an adult without further delay. Mísa keeps house together with his father and has various duties. After a failed attempt at dish-washing he goes off to the research institute where his father is working on an important invention. He is trying to produce a substance that would make poultry and other animals bigger and thus help the struggle against famine in the world, but so far he has only managed to produce a shrinking substance. After his father leaves the laboratory, Mísa mixes up different ingredients and makes the growth substance. He drinks it and instantly becomes an adult. His father, conversely, turns into a toddler after drinking the shrinking substance with his beer by mistake.
Unfortunately the movie Just Because You're Grown-Up is not yet available on HBO Max.
| Camera | Emil Bártek | Camera Operator |
| Production | Dušan Schaffer | Production Manager |
| Sound | Radomír Koutek | Sound |
| Production | Věra Hábová | Assistant Production Manager |
| Art | Zdeněk Rozkopal | Production Design |
| Writing | Pavel Vrba | Lyricist |
| Writing | Milan Šimek | Screenplay |
| Costume & Make-Up | František Čížek | Makeup & Hair |
| Camera | Jiří Kolín | Director of Photography |
| Art | Josef Šťovíček | Set Decoration |
| Editing | Antonín Štrojsa | Editor |
| Writing | Radim Cvrček | Screenplay |
| Art | František Santarius | Set Decoration |
| Costume & Make-Up | Karel Konrád | Makeup & Hair |
| Writing | Sheila Och | Story |
| Costume & Make-Up | Naďa Škapová | Costume Design |
| Sound | Bohumír Brunclík | Sound Effects |
| Directing | Zdeněk Zydroň | First Assistant Director |
| Sound | Harry Macourek | Original Music Composer |
| Crew | Josef Strnad | Dramaturgy |
| Production | Vojtěch Kunčík | Unit Production Manager |
| Directing | Radim Cvrček | Director |